The present invention relates generally to ion implantation systems, and more particularly to ion sources and profile control apparatus therefor for providing ion beams with controllable density profile in ion implantation systems.
Ion implantation systems or ion implanters are widely used to dope semiconductors with impurities in integrated circuit manufacturing, as well as in the manufacture of flat panel displays. In such systems, an ion source ionizes a desired dopant element, which is extracted from the source in the form of an ion beam of desired energy. The ion beam is then directed at the surface of the workpiece, such as a semiconductor wafer, in order to implant the workpiece with the dopant element. The ions of the beam penetrate the surface of the workpiece to form a region of desired conductivity, such as in the fabrication of transistor devices in the wafer. The implantation process is typically performed in a high vacuum process chamber which prevents dispersion of the ion beam by collisions with residual gas molecules and which minimizes the risk of contamination of the workpiece by airborne particles. A typical ion implanter includes an ion source for generating the ion beam, a beamline including a mass analysis magnet for mass resolving the ion beam, and a target chamber containing the semiconductor wafer or other substrate to be implanted by the ion beam, although flat panel display implanters typically do not include a mass analysis apparatus. For high energy implantation systems, an acceleration apparatus may be provided between the mass analysis magnet and the target chamber for accelerating the ions to high energies.
Conventional ion sources include a plasma confinement chamber having an inlet aperture for introducing a gas to be ionized into a plasma and an exit aperture opening through which the ion beam is extracted to form the plasma. One example of gas is phosphine. When phosphine is exposed to an energy source, such as energetic electrons or radio frequency (RF) energy, the phosphine can disassociate to form positively charged phosphorous (P+) ions for doping the workpiece and hydrogen ions. Typically, phosphine is introduced into the plasma confinement chamber and then exposed to the energy source to produce both phosphorous ions and hydrogen ions. The plasma comprises ions desirable for implantation into a workpiece, as well as undesirable ions which are a by-product of the dissociation and ionization processes. The phosphorous ions and the hydrogen ions are then extracted through the exit opening into the ion beam using an extraction apparatus including energized extraction electrodes. Examples of other typical dopant elements of which the source gas is comprised include phosphorous (P), arsenic (As), or Boron (B).
The dosage and energy of the implanted ions are varied according to the implantation desired for a given application. Ion dosage controls the concentration of implanted ions for a given semiconductor material. Typically, high current implanters are used for high dose implants, while medium current implanters are used for lower dosage applications. Ion energy is used to control junction depth in semiconductor devices, where the energy levels of the ions in the beam determine the degree of depth of the implanted ions. The continuing trend toward smaller and smaller semiconductor devices requires a beamline construction which serves to deliver high beam currents at low energies. The high beam current provides the necessary dosage levels, while the low energy permits shallow implants. In addition, the continuing trend toward higher device complexity requires careful control over the uniformity of implantation.
The ionization process in the ion source is achieved by excitation of electrons, which then collide with ionizable materials within the ion source chamber. This excitation has previously been accomplished using heated cathodes or RF excitation antennas. A cathode is heated so as to emit electrons which are then accelerated to sufficient energy for the ionization process, whereas an RF antenna generates electric fields that accelerate plasma electrons to sufficient energy for sustaining the ionization process. The antenna may be exposed within the plasma confinement chamber of the ion source, or may be located outside of the plasma chamber, separated by a dielectric window. The antenna is energized by an RF alternating current which induces a time varying magnetic field within the plasma confinement chamber. This magnetic field in turn induces an electric field in a region occupied by naturally occurring free electrons within the source chamber. These free electrons accelerate due to the induced electric field and collide with ionizable materials within the ion source chamber, resulting in plasma currents within the ion chamber, which are generally parallel to and opposite in direction to the electric currents in the antenna. Ions can then be extracted from the plasma chamber by one or more energizable extraction electrodes located proximate a small exit opening, so as to provide a small cross-section (relative to the size of the workpiece) ion beam.
In many ion implantation systems, a cylindrical ion beam is imparted onto a wafer target through mechanical and/or magnetic scanning, in order to provide the desired implantation thereof. Batch implanters provide for simultaneous implantation of several wafers, which are rotated through an implantation path in a controlled fashion. The ion beam is shaped according to the ion source extraction opening and subsequent shaping apparatus, such as the mass analyzer apparatus, resolving apertures, quadrupole magnets, and ion accelerators, by which a small cross-section ion beam (relative to the size of the implanted workpiece) is provided to the target wafer or wafers. The beam and/or the target are translated with respect to one another to effect a scanning of the workpiece. However, in order to reduce the complexity of such implantation systems, it is desirable to reduce the scanning mechanisms, and to provide for elongated ribbon-shaped ion beams. For a ribbon beam of sufficient longitudinal length, a single mechanical scan may be employed to implant an entire wafer, without requiring additional mechanical or magnetic raster-type scanning devices. Accordingly, it is desirable to provide ribbon beam ion sources providing an elongated ion beam with a uniform longitudinal density profile for use in such implantation systems.
The present invention is directed to ion sources and apparatus for controlling or adjusting the density profile of an elongated ion beam. The control apparatus allows refinement or tailoring of a ribbon-shaped ion beam at the source, in order to produce a desired density profile, and/or to compensate for non-uniformities in other components in an ion implantation system. The control apparatus comprises a plurality of magnet pairs proximate an elongated extraction exit through which a ribbon beam is extracted from the ion source plasma chamber. The magnet pairs in one example individually comprise electro-magnets disposed on either side of the exit opening to provide adjustable magnetic fields in a pre-extraction region so as to adjust the density profile of an extracted ion beam. The elongated ribbon beam may then be used for implantation of semiconductor wafers in a single mechanical scan, thereby simplifying the implantation system. In one implementation, the invention can be employed to provide ribbon beams up to 400 mm in length, so as to facilitate single scan implantation of 300 mm semiconductor wafer workpieces.
One aspect of the invention provides an ion source with an elongated, generally cylindrical housing with a chamber wall defining a plasma chamber. The chamber comprises an elongated exit opening extending from the plasma confinement chamber to a pre-extraction region external to the housing, through which an ion beam is extracted from the plasma confinement chamber. Density profile control apparatus is provided near the pre-extraction region, comprising a plurality of magnet pairs longitudinally spaced from one another and providing individually adjustable magnetic fields in the pre-extraction region proximate the exit opening. The magnetic fields are individually variable to allow selective adjustment of the density profile associated with an extracted ion beam.
The magnet pairs may individually comprise a first magnet located on the ion source housing above the exit opening and a second magnet located on the housing below the exit opening. The first magnet provides a magnetic pole of one magnetic polarity facing the second magnet and the second magnet provides an opposite magnetic pole facing the first magnet, such that the magnets in each pair cooperate to create an adjustable magnetic field in the pre-extraction region proximate the exit opening. In one implementation, the magnets pairs comprise electro-magnets having energizable windings through which current may be conducted so as to provide the adjustable magnetic field between the first and second magnets. The control apparatus may further comprise a power source providing current to the first and second magnets of the magnet pairs in a controlled fashion, and a control system for individually controlling the currents supplied to the magnet pairs. In this manner, the magnetic fields produced by the individual magnet pairs in the pre-extraction region may be adjusted or varied according to a desired density profile for the ion beam being extracted from the plasma confinement chamber.
Another aspect of the invention involves a method for controlling a density profile associated with an elongated ion beam in an ion implantation system. The methodology comprises providing a plurality of magnet pairs longitudinally spaced from one another near an exit opening in a plasma confinement chamber, and adjusting the magnetic fields in the pre-extraction region using the magnet pairs. In this manner, the density profile associated with an extracted ion beam may be adjusted.
To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which the principles of the invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the drawings.